1. Field of the Invention
The present invention relates to one-time programmable (OTP) devices used in microelectronics, and more specifically to such devices using semiconductor fuses.
2. Discussion of the Related Art
OTP devices using semiconductor fuses find many applications in microelectronics. They are, for example, used to short-circuit resistors in a chain of series resistors to adjust a reference voltage in an integrated circuit. Such OTP devices may also be used to permanently store, in an integrated circuit, after its manufacturing, data enabling identification of the circuit from the outside, or required by the circuit operation.
FIG. 1 shows a portion 10 of an integrated circuit including a conventional OTP device 20. Device 20 includes a logic gate 22 and a fuse 24. Logic gate 22 receives a selection signal S from a selection device 26 and a programming signal P. Fuse 24 is arranged between the output F of logic gate 22 and an output O. A fuse here designates a device including a fusible element, means for fusing the fusible element, generally by over-current, and means for generating a binary output O indicating whether the fusible element has fused or not. The structure of this device is well known in the art and will not be detailed herein.
To program device 20, signal S is activated to select device 20, and programming signal P is activated. Then, logic gate 22 generates a signal F which causes the fusing of fuse 24. Selection device 26 is generally controlled by an external control signal C.
A conventional device such as shown in FIG. 1 has several significant disadvantages. It may occur that fuse 24 does not completely fuse when receiving signal F. Thus, in some cases, the fuse may generate an incorrect value after programming. In practice, the probability of a failure is approximately 1%. When a fuse has not completely fused, it is difficult to have it completely fuse by a second programming, since its resistance is then too high to reach the fusing energy with the admissible programming voltage.
Further, a properly fused, that is, non-conductive fuse 24 may become conductive again in the course of time, by recombination of the fused elements. The probability of such a recombination is approximately 1%.
Moreover, circuit 10 may be submitted to electrostatic discharges which can, in some conditions, simultaneously activate signals P and S. Element 20 may under such circumstances be untimely programmed.
Eventually, when using an OTP device such as that in FIG. 1 for settings, for example, to short-circuit resistors in a chain of resistors enabling determination of a reference voltage, it may be desirable to have a way of knowing the effect of the programming of device 20 before effectively programming it.
Because of the combination of the above-mentioned disadvantages, those skilled in the art consider semiconductor fuse devices as unreliable. As a result, for example, such devices are not used in applications for which a high programming reliability is indispensable. In such applications, more complex programming means are then used.
An object of the present invention is to provide a fuse one-time programmable device, the programming of which is particularly reliable.
Another object of the present invention is to provide such a device which is protected from electrostatic discharges.
Another object of the present invention is to provide such a device associated with means for knowing the effect of the programming before proceeding to the programming.
To achieve these objects, as well as others, the present invention provides an one-time programmable device including three fuses connected in parallel to a logic element which determines that the device is programmed when at least one of the fuses is open.
According to an embodiment of the present invention, the device includes means for checking the programming of the fuses and declaring the device to be not suitable if less than two fuses are open.
According to an embodiment of the present invention, the device is programmable by a programming signal.
According to an embodiment of the present invention, the logic element is provided to indicate a programmed state of the device when it receives a test control signal.
According to an embodiment of the present invention, the device includes a circuit for preventing the programming signal to be generated by an electrostatic discharge.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.